Risk of bias assessment and certainty of evidence assessment
Figure S2 presents the risk of bias assessment for individual trials. Thirteen trials were deemed low risk of bias and 6 trials were deemed high risk of bias. Two trials were at high risk of random sequence generation, and 10 trials were at unclear risk of random sequence generation. All trials were unclear in describing the methods used to conceal allocation and therefore were at unclear risk of selection bias. Three trials were at high risk of performance bias owing to inadequate blinding of participants, and 16 were at unclear risk of performance bias. Four trials were at high risk of detection bias owing to inadequately blinding outcome assessment, and another 15 trials were at unclear risk of detection bias. All studies were at low risk of attrition bias and reporting bias. The overall certainty of evidence by GRADE was summarized in Table 1. Overall, moderate certainty of evidence was found for second-degree perineal laceration, while the evidence for perineal trauma and severe perineal laceration was rated as low certainty. Very low certainty of evidence was found for episiotomy, first-perineal laceration, intact perineum, duration of the second-stage labor.
Incidence of perineal